The present invention relates generally to automatic white adjusting circuits for television cameras, and more particularly to an automatic white adjusting circuit for a television camera in which the adjustment of the white is performed automatically by the simple operation of aiming the television camera on a white surface.
Conventionally, in a high-grade television camera for business use, an automatic white adjusting circuit is provided to automatically adjust the white (correct the color temperature) so that the tint of an output signal of the color television camera does not change due to the color temperature of the illumination light source. This automatic white adjusting circuit is based on the theory that, upon picking-up an image of a white object, the three primary colors of red, green, and blue lights must be of the same level, and it adjusts the respective output signals of the three primary colors so that the three levels become the same.
There have been many kinds of conventional automatic white adjusting circuits in which an analog or a digital system is used. However, they all suffered disadvantages in that they all possessed a plurality of operating buttons which were troublesome to operate, and in the analog systems, the disadvantage was that the automatic white adjusting circuit as a whole became costly because of the high cost of the analog memories. Therefore, it was not possible to apply the automatic white adjusting circuit of this television camera for business use to the desired television camera for home use.
Furthermore, the television camera for business use is designed on the assumption that it will be operated by a professional operator, and requires the adjustment of the white every time the voltage source is applied to the television camera, thus also in this respect it is, not applicable to the television camera for home use.
A conventional portable color television camera is supplied with voltage from a voltage source of a portable video tape recorder. This type of portable video tape recorder shuts the voltage source off when the tape travel is put in the stop mode to reduce the power consumption. Accordingly, when the video tape recorder is put in the stop mode, the voltage source of the television camera also is shut off.
Therefore, in the automatic white adjusting circuit using digital memories, it suffers disadvantages in that the content of the digital memory is destroyed when the voltage source is shut off, requiring the adjustment of the white on every image-picking-up occasion, the operation being troublesome, and not being able to take pictures in the preferable timing.
To overcome these disadvantages, the television camera could possibly be provided with its own battery, so that the content of the digital memory can be kept in the same state even when the video tape recorder is in the stop mode. However, by providing the television camera with its own battery, both the weight and size increase, thus resulting in the disadvantages that the reduction of weight and size demanded for the portable television camera cannot be met.
On the other hand, when the automatic white adjusting circuit is constructed by using analog memories, the television camera does not require its own battery since the analog memory content does not change when the voltage source is put on or off. But the content of the analog memory is slowly destroyed with time by natural discharge, thus requiring a circuit to reduce this natural discharge, resulting in high cost, suffering the disadvantage that it is not suitable for television cameras for home use demanding low cost.
Therefore, an automatic white adjusting circuit which is especially effective when applied to a color television camera for home use, comprising a simple circuit construction and in which the white is automatically adjusted by a simple operation, was highly desired.
In the color television systems presently being used such as the NTSC, PAL, or SECAM systems, they are standardized to transmit a luminance signal and two color difference signals (I-and Q-signals in the NTSC system, and (R-Y) and (B-Y) signals in the PAL system). The white is obtained when the two color difference signals are zero, and the color television camera for home use comprises a circuit for generating the above color difference signals.
Upon picking-up of an image of an entirely white object by a television camera, the color difference signal is zero when the white adjustment matches with the color of the illumination light source, and the output of the above color difference signal generating circuit leans to the positive or negative side when the white adjustment does not match with the color of the illumination source. To adjust this leaning to the positive or negative side to zero, the primary color signal from the light element of any of the two primary colors of the three primary colors red (R), green (G), or blue (B) can be increased or decreased.
In the circuit for generating the color difference signals (R-Y) and (B-Y), for example, the amplification rate of the primary color signals or R and B can be gradually decreased when the generating output signal increases towards the positive side, and the amplification rate of the primary color signals of R and B (referred as R-signal and B-signal hereinafter) can be gradually increased when the generating output signal increases towards the negative side, respectively. The (R-Y) and R signals, and the (B-Y) and B signals, respectively, do not coincide completely with each other, but since the R signal contributes most to the (R-Y) signal and the B signal contributes most to the (B-Y) signal, the levels of the primary color signals R and B can be adjusted accordingly to adjust the two color difference signals (R-Y) and (B-Y), respectively, to zero.
However, upon actual operation, there is no problem when an entirely white surface exists under the illumination light source, but outdoors, in most cases, a white plate which is an object of an entirely white surface is not on hand. When a scenery having each color in a balanced manner is picked-up by a television camera in this case, positive or negative signals appear in the signal obtained from the color difference signal generating circuit in the television camera, the positive or negative signals having the color difference signal obtained upon picking-up of a white plate under that particular illumination light source as its center, and in the majority of occasions, the average value of the color difference signal at that time can be taken as the color difference signal obtained upon picking-up the image of a white plate under that illumination light source.
There are also times when the white surface is too small for the enlargement to the full scale of the image picking-up field of vision, and in this case, it is convenient to use the color difference signal relative to one portion of the picture, such as the central portion of the picture.